Mixed-function oxidases containing different forms of cytochrome P-450 are the key enzymes that metabolize a wide variety of drugs, chemicals and carcinogens. The focus of this project is the utilization of specific inhibitors and inducers of aryl hydrocarbon hydroxylase (AHH) to probe the mutiplicity, diversity, and different catalytic properties of the cytochromes P-450. The form of AHH that is induced by polycyclic hydrocarbons is strongly inhibited by 7,8-benzoflavone, while this compound inhibits weakly or stimulates the form of AHH found in untreated or phenobarbital (PB)-treated rats. In contrast, 1-maackiain acetate greatly inhibits the AHH from untreated or PB-treated rats and inhibits weakly or stimulates the form of AHH induced by polycyclic hydrocarbons. Therefore, 7,8-benzoflavone and 1-maackiain acetate are inhibitors that are effective against different forms of AHH. We are using these inhibitors to probe the active catalytic sites of the multiple forms of cytochrome P-450. Also, other compounds, i.e., medicarpin and maackiain, are being studied and their structural and inhibitory effects on cytochrome P-450 are being determined using microsomes and purified methylcholanthrene and PB-induced P-450s.